Resolution-dependent and color-dependent print masking

ABSTRACT

At least two different printmasks are used for different printing devices--such as inkjet pens--that operate concurrently, or different printing steps that proceed concurrently, to produce respective image swaths in a single, pixel-based printing machine. In one form of the invention, the different printing devices produce different respective pixel-row pitches (related to resolution) on a printing medium, and the different printmasks help to minimize adverse patterning effects that result from interaction of the different pitches with dot-placement errors. The different pitches on the print medium may be provided through different pitches of marking devices (for example, the number of inkjet nozzles per unit distance along a pen) or in other ways. In another form of the invention the different printing devices are provided with different respective liquid-base colorants, to mark in different colors on a printing medium, and the different masks facilitate physical separation of the different colorants to promote drying. Earlier-applied colorants have more time to dry and penetrate before later adjacent or superposed application of other colorants improving print attributes such as bleed, offset, cockle, curl, overall drying time and throughput.

RELATED PATENT DOCUMENTS

This is a continuation of application Ser. No. 08/399,007 filed on Mar.6, 1995, now U.S. Pat. No. 5,883,644 which is a continuation-in-part ofU.S. application Ser. No. 08/145,261, entitled "MIXED RESOLUTIONPRINTING FOR COLOR AND MONOCHROME PRINTERS" and filed Oct. 29, 1993, nowU.S. Pat. No. 5,949,453, in the names of Donald G. Harris; Majid Azmoonand Gary M. Nobel--and commonly owned herewith, and incorporated hereinby reference.

This document also relates to the following copending applications whichare commonly owned herewith, and which are incorporated herein byreference: Ser. No. 08/145,367, "INTERCONNECT SCHEME FOR MOUNTINGDIFFERENTLY CONFIGURED PRINTHEADS IN THE SAME CARRIAGE", filed Oct. 29,1993 in the names of Gary M. Nobel, et al.; Ser. No. 08/56,345,"ELECTRICAL INTERCONNECT SYSTEM FOR A PRINTER", filed Apr. 30, 1993, inthe names of Arthur K. Wilson, et al.; Ser. No. 08/55,618, "MODULARCARRIAGE ASSEMBLY FOR AN INKJET PRINTER", filed Apr. 30, 1993, in thenames of Arthur K. Wilson, et al.; Ser. No. 08/56,009, "WIPING STRUCTUREFOR CLEANING ELECTRICAL CONTACTS FOR A PRINTER AND INK CARTRIDGE", filedApr. 30, 1993, in the names of Corrina A. E. Hall, et al.; Ser. No.08/56,961, "METHOD AND DEVICE FOR PREVENTING UNINTENDED USE OF PRINTCARTRIDGES", filed May 3, 1993, in the names of Jeffrey A. Thornan, etal.; Ser. No. 08/57,241, "SIDE BIASED DATUM SCHEME FOR INKJET CARTRIDGEAND CARRIAGE", filed Apr. 30, 1993, in the names of David W. Swanson, etal.; Ser. No. 07/958,833, "PRINTHEAD WITH REDUCED INTERCONNECTIONS TO APRINTER", filed Oct. 8, 1992, in the names of Michael B. Saunders, etal.; Ser. No. 08/056,263, entitled "INKING FOR COLOR-INKJET PRINTERS,USING NONINTEGRAL DROP AVERAGES, MEDIA-VARYING INKING, OR MORE THAN TWODROPS PER PIXEL" and filed Apr. 30, 1993, in the names of Ronald A.Askeland, Catherine B. Hunt, Keshava A. Prasad, Corrina A. E. Hall, MarkStephen Hickman, Lance Cleveland, and William J. Allen; Ser. No.08/056,633, entitled "MAXIMUM-DIAGONAL PRINT MASK AND MULTIPASS PRINTINGMODES, FOR HIGH QUALITY AND HIGH THROUGHPUT WITH LIQUID-BASE INKS" andfiled Apr. 30, 1993, in the name of Lance Cleveland; and attorneydockets 1094941 and 1094942, later designated as Ser. Nos. 08/396,854and 08/397,295, filed generally contemporaneously herewith in the nameof Mark Stephen Hickman or Mark Stephen Hickman et al. and entitledrespectively "COLOR INK JET PRINTING METHOD" and "COLOR INK JET PRINTINGMECHANISM WITH ELONGATED BLACK NOZZLE ARRAY AND METHOD OF OPERATION".

BACKGROUND

1. Field of the Invention

This invention relates generally to machines and procedures for printingtext or graphics on a printing medium such as paper, transparency stock,or other glossy media; and more particularly to a scanningthermal-inkjet machine and method that construct text or images fromindividual ink spots created on a printing medium, in a two-dimensionalpixel array. The invention employs print-mode techniques to optimizeimage quality and operating time, and also to minimize distortion of theimage and of the printing medium.

2. Related Art

In a multiple-pen printer, it is important to as economically and simplyas possible maximize both the output quality of a printed page and thespeed at which that output can be obtained.

In a printer mechanism, the output quality of a printed page is afunction of printhead resolution. The finer the resolution, the betterthe print quality.

Also, in a swath printer (e.g., one employing a scanning carriage with apen capable of printing multiple pixel rows concurrently) the speed atwhich the output can be obtained is a function of the height of theswath which is covered by the printhead.

In multipen printers, until making of the invention covered by theHarris et al. document mentioned above, each pen had the same resolutionand usually the same swath width. This meant that increasing resolutionof any part of the system would require scaling up all the supportingstructure, mechanics and electronics to support the resolution of theentire set of pens.

Thus heretofore in designing printing machines it has been necessary toconfront difficult choices between high speed, high resolution and otherprint-quality characteristics, and economy. The aforementioned Harris,Azmoon and Nobel document, however, teaches that many such choices canbe advantageously sidestepped.

Harris et al. provide within a single printer plural writing devices ofdifferent color, speed, swath height and resolution. The printer isprogrammed to operate the writing devices in a way that takes advantageof the swath height, speed and resolution characteristics of eachwriting device to obtain an enhanced mix of speed, resolution andeconomy.

For example, if one pen creates a relatively tall, high-resolution swathand is loaded with black ink, that pen can be used for relatively rapidthroughput of black text or graphics alone. The printer is therebycapable of serving in the stead of a relatively fast, high-resolutionblack-text or black-only-graphics printer.

The same pen can be used for relatively rapid throughput of the blackcomponent of color images. Other pens, loaded with color inks--oranother single pen capable of discharging different color inks--in thesame printer can be used to form the chromatic components of colorimages. Thus addition of a relatively small amount of hardware enablesthe printer to do color work as well as fast, high-resolution blackprinting.

Providing just one pen of desired higher resolution and greater swathheight--and providing in the same set other pens of lower resolution andlesser swath height--creates a plural-resolution, plural-swath-widthsystem. Such a hybrid system is far less expensive than the full, majorhardware scale-up, mentioned above, that would be needed for anall-high-resolution, all-high-swath-height printer.

Thus in the interest of economy the color pen or pens can be limited incapability to creation of a relatively shallow, lower-resolution swath.Most interestingly, such economy is not severely deleterious to printedresults, inasmuch as the perceptual capability of the human eye isrelatively insensitive to detail in chromatic features. Moreover,chromatic colors in a large fraction of practical cases (particularly inbusiness graphics) are used only to provide color fill in relativelylarge, uniform fields.

Thus the invention of Harris, Azmoon and Nobel incorporateplural-resolution capabilities directly into the printer printheads,expanding the capabilities of the printer to achieve high-qualityprinting as well as greater throughput. Their invention decreasesresearch and development costs as well as decreasing the time forbringing higher-resolution printers to market.

Harris et al. provide a color printer having one basic printheadresolution for color printing and a different basic printhead resolutionfor monochrome printing such as black printing. In a preferred form, ahigher basic printhead resolution is provided for monochrome printing(particularly black) and a lower for color (particularly cyan, magentaand yellow). They integrate these black and color printing componentsinto the same printing mechanism, providing composite printing ofhigher-resolution black and lower-resolution color concurrently.

Harris et al. furthermore provide increased throughput for thehigher-resolution monochrome component of the color printer. In theirpreferred form, a taller-swath monochrome printhead such as ahigh-resolution black printhead which produces approximately dots ofsuitable size for spacing at 23.6 dots per mm (600 dots per inch, or"dpi") is mounted on the same carriage as shallower-swath colorprintheads such as lower-resolution cyan, magenta and yellow printheadswhich produce approximately 11.8 dots/mm-sized (300-dpi-sized) printoutdots.

The taller-swath black printhead has overlapping printing alignment withall of the shallower-swath color printheads. The taller-swath blackprinthead has a three-hundred-nozzle swath with a nozzle pitch of about0.042 mm (1/600 inch), to create a swath of approximately 12.7 mm(one-half inch), and the shallower-swath color printheads each have ahundred-nozzle swath with a nozzle pitch of about 0.085 mm (1/300 inch)to create a swath of approximately 8.5 mm (one-third inch).

The availability of plural writing devices of different character in asingle printer, however, does not--in and of itself--cure every problemof swath-based printing technology. Some such problems that remain areoutlined below.

Furthermore, under certain circumstances the use of plural writingdevices of different character in a single printer can introduceundesired subtle patterning effects, of a sort not found in printersusing only matched writing devices. In particular, when printing devicesof different resolution are used together, dot-placement errors candifferently affect printing at the different resolutions, degradingprint quality at one or another resolution.

The present invention relates to certain of the remaining problems, andalso certain subtle degradations that are inherent in the use ofdifferent plural writing devices together.

(a) Ink-flux effects--To achieve vivid colors in inkjet printing withaqueous inks, and to substantially fill the white space betweenaddressable pixel locations, ample quantities of ink must be deposited.Doing so, however, requires subsequent removal of the water base--byevaporation (and, for some printing media, absorption)--and this dryingstep can be unduly time consuming.

In addition, if a large amount of ink is put down all at substantiallythe same time, within each section of an image, related adversebulk-colorant effects arise: so-called "bleed" of one color into another(particularly noticeable at color boundaries that should be sharp),"blocking" or offset of colorant in one printed image onto the back ofan adjacent sheet with consequent sticking of the two sheets together(or of one sheet to pieces of the apparatus or to slipcovers used toprotect the imaged sheet), and "cockle" or puckering of the printingmedium.

These problems are well known in the art. Various techniques are knownfor use together to moderate these adverse drying-time effects and bulk-or gross-colorant effects.

(b) Staggered pens--Colors can be separated during printing by use ofstaggered, i.e. vertically offset, pens. Such an approach is relativelyundesirable because it requires a bigger carriage and a bigger printer,and also introduces additional complexity in mutual alignment of theseveral pens.

(c) Prior heat-application techniques--Among these techniques is heatingthe inked medium to accelerate evaporation of the water base or carrier.Heating, however, has limitations of its own; and in turn creates otherdifficulties due to heat-induced deformation of the printing medium.

Glossy stock warps severely in response to heat, and transparencies toocan tolerate somewhat less heating than ordinary paper. Accordingly,heating has provided only limited improvement of drying characteristicsfor these plastic media.

As to paper, the application of heat and ink causes dimensional changesthat affect the quality of the image or graphic. Specifically, it hasbeen found preferable to precondition the paper by application of heatbefore contact of the ink; preheating, however, causes loss of moisturecontent and resultant shrinking of the paper fibers. Shrinkage iscommonly nonuniform and creates gross distortions of the medium andnaturally its image. Through closer control of the printing medium andthe image segments near the ends of the pages, such problems have beenmitigated but not entirely eliminated.

(d) Prior print-mode techniques--Another useful technique is laying downin each pass of the pen only a fraction of the total ink required ineach section of the image--so that any areas left white in each pass arefilled in by one or more later passes. This tends to control bleed,blocking and cockle by reducing the amount of liquid that is all on thepage at any given time, and also may facilitate shortening of dryingtime.

Print modes have been designed to minimize the conspicuousness of imagedistortions arising in various ways. The aforementioned patent documentof Cleveland presents an extensive discussion of some print modes andthe problems they attack; a brief discussion appears shortly in anothersubsection hereunder.

The specific partial-inking pattern employed in each pass, and the wayin which these different patterns add up to a single fully inked image,is known as a "print mode". Heretofore print modes have beensubstantially the-same for all printheads (for example, all pens) usedat any one time in each printer--and accordingly have been substantiallythe same for all colors printed concurrently in a given printer.

(e) "Concurrent" printing--As used in this document, the term"concurrently" is intended to encompass ongoing continuing operations ofa generally unitary printing machine, such as for example (1) printingone color during one direction of scanning of a pen carriage, andforthwith printing another color in another direction of scanning; or(2) printing one group, e.g. some specified number of rows, of pixels inone set of passes across a print medium, and then printing another groupof pixels that are interspersed among the first group, in another set ofpasses. Thus the word "concurrently" encompasses, but does not require,printing "simultaneously".

As used in this document, however, the word "concurrently" excludes suchoperations as printing one element (one color, or one group of pixels)for an entire page, or for an entire image, and then printing anotherelement (another color or another group of pixels) for the same entirepage or image. This sort of printing is excluded by the term"concurrently" whether the successive elements are printed on onegenerally unitary printing machine or on more than one such machine.

(f) Resolution--Furthermore printheads used together have had commonsize and provided common resolution. Accordingly partial-inking patternshave been substantially the same for the resolutions of all pens.

In this regard it is important to have a clear understanding of what ismeant by the resolution of a pen, for purposes of the present document.High quality printers are typically characterized by numbers indicatingtheir resolution in dots per millimeter (dots/mm) or dots per inch(dpi). This resolution is usually described by a pair of numbers, in thecontext of a two-dimensional coordinate system--where one numberindicates the resolution along the x-axis (as used herein, x-axis meansthe axis of carriage scanning for a swath printer), and another numberindicates the resolution in the y-axis (as used herein, y-axis means theaxis of printing-medium advance for a swath printer). Thus, a resolutionof 11.8/11.8 dots/mm (300/300 dpi) generally indicates a carriage-scanaxis resolution of 11.8 dots per millimeter (300 dots per inch) and aprinting-medium-advance axis resolution of 11.8 dots per millimeter (300dots per inch).

The term "resolution" means ability to resolve or separate--usually toseparate visually--two image elements or details. In one sense,resolution of a printhead is primarily determined by the actual printoutdot size as it appears in a printout, since perceptual separation isdifficult for two large dots even if their centers are geometricallydisplaced, by some distance smaller than their radii. So in one idealtheoretical world, an 11.8 dot/mm (300 dpi) printhead is presumed toproduce a printout dot size which is approximately 0.085 mm (1/300 inch)in diameter.

In another and more fundamental sense, however, dot size can besubordinated to center-to-center spacing, since resolution finer thancenter-to-center spacing is a technical impossibility regardless of dotsize. Therefore, various common language usages have developed whichdefine resolution in other closely related terms. For example, theresolution of a printhead is often identified by its nozzle pitch (i.e.,the distance between adjacent nozzles on a printhead), and a print moderesolution is often identified by its pixel addressability (i.e., thedistance between adjacent pixels in a printout).

There are several print mode techniques for enhancing the print-qualitycharacteristics of a printhead. For example, an 11.8 dot/mm (1/300 inch)nozzle-pitch printhead can be used to create a 23.6 dot/mm (600pixel/inch) printout along the print-medium-advance axis by changing theincremental advance distance of the medium at the end of a swath andthen employing a multipass print mode.

Such a system which provides printout pitch that is finer than thehardware pitch is sometimes called an "addressable" fine-pitch orhigh-resolution system. As another example, an 11.8 dot/mm (1/300 inch)nozzle-pitch printhead could be used to create a 23.6 dot/mm (600pixel/inch) printout along the carriage-scan axis by suitably choosingthe firing frequency of the printhead or the carriage scan speed, orboth.

Implementing these different print modes, however, is rather complicatedand requires sophisticated programming techniques, precisely engineeredmechanical parts, and many circuit components. Moreover, the printquality of a lower-resolution machine which has a 23.6 dot/mm (600 dpi)"addressable" print mode is not as good as the print quality of a true23.6 dot/mm-resolution (600-dpi-resolution) machine in which bothsmallest dot size and addressability are each equal to 23.6 dots/mm (600dpi).

Some print modes such as square or rectangular checkerboard-likepatterns tend to create objectionable moire effects when frequencies orharmonics generated within the patterns are close to the frequencies orharmonics of interacting subsystems. Such interfering frequencies mayarise, for example, in dithering subsystems sometimes used to helpcontrol the paper advance or the pen speed.

(g) Known technology of print modes: general introduction --Oneparticularly simple way to divide up a desired amount of ink into morethan one pen pass is the checkerboard pattern mentioned above: everyother pixel location is printed on one pass, and then the blanks arefilled in on the next pass.

To avoid horizontal "banding" problems (and sometimes minimize the moirepatterns) discussed above, a print mode may be constructed so that thepaper advances between each initial-swath scan of the pen and thecorresponding fill-swath scan or scans. In fact this can be done in sucha way that each pen scan functions in part as an initial-swath scan (forone portion of the printing medium) and in part as a fill-swath scan.

Once again this technique tends to distribute rather than accumulateprint-mechanism error that is impossible or expensive to reduce. Theresult is to minimize the conspicuousness of--or, in simpler terms, tohide--the error at minimal cost.

(h) Print masks vs. inking locations--Masking relates to addressabilityof pixel positions in each operation of a printing device (for example,each pass of a transversely scanning pen). Actual inking (or actualaddressing) of particular pixels is thus a very different matter frommasking.

Actual inking depends upon not only (1) addressability but also (2) thedesired-image data which a particular user supplies to the printer--ineffect, the image detail which the user wishes to see in the vicinity ofeach pixel--and (3) various special procedures, known as "rendition",employed to translate desired image details into a pattern of ink dotswhich the printer can produce.

Thus desired-image data and rendition procedures typically preventactual inking of most colors (or, in many systems, almost all colors) ineach pixel position. This is true even for a pixel which is addressablefor all, or almost all, colors if only print masking alone isconsidered.

Conversely, in any given operation of a given printing device (e.g., penpass) even if image data and rendition call for printing a particularcolor at a particular pixel, that color may not be printed at thatpixel. Depending on the masking scheme in use, that color may have beenprinted there already in a previous operation, or may be reserved forprinting there in a later operation, of that same (or another) printingdevice.

(i) Print masks vs. unit quantity of colorant--In some devicesheretofore, different quantities (for example, a different number ofdrops) of ink are provided for different colors. This is particularlytrue for different types of printing media.

Thus in printing on transparency stock (plastic sheets used for making,as an example, masters that can be projected on a screen by an overheadprojector) it is necessary to provide much more colorant than whenprinting on paper. Similarly glossy stock (plastic-coated paper popularas cover sheets for reports or looseleaf books) requires more colorantthan ordinary paper requires, to produce a like impression of colorvividness.

Variable inking in such situations, whether for different colors ordifferent media, or different crosscombinations of the two, is producedin rendition. Here too, this is not a matter of masking--which relatesonly to assigning particular color-to-pixel applications to particularprinting-device operations. In all such situations, the maskingheretofore is the same for all printing devices and all colors withinany given printer.

The same is true whether the unit quantity for each color is one dot ofcolorant, or plural dots, or even fractional dots (as described forexample in the aforementioned patent document of Askeland et al.). It isalso true whether printing is binary (go or no-go, for each color) orplural-level (for instance any number of color-quantity units from zeroto 2^(n), for each color, where n is the number of data bits in thecolor-specification system). In all these variants, masking is common toall colors and all printing devices used concurrently within any oneprinter.

(j) Print-mode masks: space- and sweep-rotated, and autorotating--Thepattern used in printing each nozzle section is known as the "print-modemask". The term "print mode" is more general, usually encompassing adescription of a mask, the number of passes required to reach fulldensity and the number of drops per pixel defining "full density".

Operating parameters can be selected in such a way that, in effect,rotation occurs even though the pen pattern is consistent over the wholepen array and is never changed between passes. Figuratively speakingthis can be regarded as "automatic" rotation or simply "autorotation".

The Cleveland patent document mentioned earlier discusses thesetechniques at greater length.

(k) Conclusion--As pointed out above, availability of plural writingdevices of different character in a single printer has left someproblems remaining to be solved in swath-based printing technology. Asalso noted above, use of plural writing devices of different characterin a single printer can itself introduce subtle undesired effects notfound in printers using matched writing devices exclusively.

Thus important aspects of the technology used in the field of theinvention are amenable to useful refinement.

SUMMARY OF THE DISCLOSURE

The present invention introduces such refinement. In its preferredembodiments, the present invention has several aspects or facets thatcan be used independently, although they are preferably employedtogether to optimize their benefits.

The invention relates to exploiting the presence of different writingdevices in a single printer to ameliorate certain of the remainingproblems mentioned above, or certain effects newly introduced by usingwriting devices of different character together in one printer.

In preferred embodiments of a first facet or aspect of its aspects, theinvention is a mixed-masking printer for forming images as an assemblageof ink dots at pixel locations on a printing medium.

The printer includes some means for holding the print medium. Any of agreat variety of means may be used for this purpose; therefore and forpurposes of breadth and generality these means will be called the"print-medium holding means".

In addition the printer includes some means for operating to print animage swath on a particular region of such medium. Again for generalityand breadth these means will be called simply the "first printingmeans".

The printer also has some means, distinct from the first means, forconcurrently operating to print an image swath on the same particularregion of such medium. These will be called the "second printing means".(As stated earlier this language encompasses printing of the secondswath height simultaneously, or within the ongoing continuous operationof a single printing machine--but not by successive passes of one ormore machines over an entire page or an entire image.)

Also the printer includes some means for supporting the first and secondprinting means together, relative to the print-medium holding means.These means will be called the "common supporting means" or more simply"supporting means".

The printer furthermore includes some means for controlling the firstprinting means to impose on the first image swath a first printmask.These means will be called the "first controlling means" or simply the"controlling means".

Finally the printer includes some means for concurrently controlling thesecond printing means to impose on the second image swath a secondprintmask that is different from the first printmask. These means willbe called the "second controlling means" or for perhaps greater claritythe "concurrently-controlling means".

The foregoing may constitute a description or definition of the firstfacet of the invention in its broadest or most general form. Even inthis general form, however, it can be seen that this aspect of theinvention significantly mitigate the difficulties left unresolved in theart.

In particular, through use of different printmasking for differentprinting means, the invention enables the taking of countermeasures toavoid dot-placement errors that could otherwise affect the printingdifferently depending on resolution produced by different printingmeans. Alternatively (or in addition), the invention takes advantage ofthe additional degree of freedom to enable reduction of drying time,bleed, paper curl-and cockle, image deformation, and the several otherrelated limitations discussed earlier.

Although this aspect of the invention in its broad form thus representsa significant advance in the art, it is preferably practiced inconjunction with certain other features or characteristics that furtherenhance enjoyment of overall benefits.

For example, it is preferred that the first printing means produce afirst particular pixel-row pitch on the medium; and that the secondprinting means produce a second particular pixel-row pitch, on themedium, that is different from the first pitch. (This parameter"pixel-row pitch, on the medium" is tantamount to the actual effectiveprinted resolution in the swath-height direction on the printingmedium--as distinguished from the pitch of individual marking-elementdevices on the printing means, e.g. nozzles on a pen. In various kindsof mechanical systems the printed pitch differs from the marking-elementpitch.)

In particular the second pitch is preferably twice as fine as the firstpitch. In this case it is also preferable--in one particular printingenvironment--that one printmask-cell height for the second printingelement contain three-quarters as many fine-pitch units as oneprintmask-cell height for the first printing element contains.Alternatively it is preferred that a printmask cell for the secondprinting element be three-quarters as tall as a printmask cell for thefirst printing element.

In one particularly preferred combination of parameters (still relatedto the mixed-print-density preference mentioned above), the printmaskfor the first printing element includes a first checkerboard pattern inwhich:

each unit square is three fine-resolution pixels on a side, and

alternate unit squares are addressed in alternate operations of thefirst printing means; and

the printmask for the second printing element include a second,elongated checkerboard pattern in which:

each unit rectangle is two coarse-resolution pixels tall and onecoarse-resolution pixel wide, and

alternate unit rectangles are addressed in alternate operations of thesecond printing means.

Also still in connection with mixed printing density it is preferredthat the first pitch and the second pitch stand in the ratio of twosmall integers. Thus common masking is possible, though not preferred.

Still another preference related to mixed print density is that theprinting means include scanning inkjet pens having ink-ejecting nozzles;and for each pen the nozzles are disposed in respective arrays ofregular pitch. In this connection preferably the pixel-row pitch foreach printing means is related to the nozzle pitch of the correspondingpen.

It is also preferred that the first printing means print in a firstparticular color; and the second printing means print in a secondparticular color. More specifically, it is preferred that the firstparticular color be a chromatic color and the second particular color beeither black or a second chromatic color.

Preferably too the printmask cell for the second printing means is wideror taller (or both wider and taller) than the printmask cell for thefirst printing means. Some more-specific preferred printmask patternsare these three patterns:

(1) one preferred pattern

The first printing means have first and second segments;

the second printing means have first and second segments that arerespectively aligned with the first and second segments of the firstprinting means;

at least sometimes the first printmask addresses the first segment ofthe first printing means at pixel columns for which the second printmaskaddresses the second segment of the second printing means; and

at least sometimes the first printmask addresses the second segment ofthe first printing means at pixel columns for which the second printmaskaddresses the first segment of the second printing means.

(2) a second preferred pattern

At least sometimes the first printmask addresses a top half of the firstprinting means at pixel columns for which the second printmask addressesa bottom half of the second printing means.

(3) a third preferred pattern

The printmask for the first printing element comprises a first,elongated checkerboard pattern in which:

each unit rectangle is two pixels tall and one pixel wide, and

alternate unit rectangles are addressed in alternate operations of thefirst printing means; and

the printmask for the second printing element comprises a secondcheckerboard pattern--in which:

each unit square is two pixels on a side, and

alternate unit squares are addressed in alternate operations of thesecond printing means.

Preferably, in relation to the most general and broad form of theinvention introduced above, the first printing means have a firstparticular printing-element pitch and the second printing means have asecond particular printing-element pitch that is different from thefirst pitch. Preferably the printing is not only concurrent but alsosimultaneous.

Preferably the first and second printing means are scanning pens; andthe common supporting means include a carriage that supports the firstand second printing means in common, for scanning together across themedium. Preferably the first printing means include either pluraldiscrete pens or a substantially unitary pen having plural ink-supplychambers; and the second printing means include at least one other pen.

In preferred embodiments of a second of its main facets or aspects, theinvention is a method of forming images as an assemblage of ink dots atpixel locations on a printing medium. This method includes the step ofprinting on a particular region of the medium, using a first printmask,an image swath of a first particular character.

The method also includes the step of concurrently printing on the sameparticular region of the medium, using a second printmask that isdifferent from the first printmask, an image swath of a secondparticular character which is different from the first particularcharacter.

The first and second particular image-swath character differ withrespect to either resolution or color.

Benefits of this second, method aspect of the invention are enjoyed thatare related to those associated with the printer aspect of theinvention. By including a different masking step or steps for differentprinting means, the invention can facilitate reductions in drying time,bleed, paper curl and cockle, image deformation, etc. It can also helpavoid dot-placement errors that could otherwise affect the printingdifferently depending on resolution produced by different printingmeans.

Nonetheless still further advantages may be gained by incorporatingother features or characteristics. For instance it is preferred that thefirst and second particular image-swath character comprise a first andsecond particular color respectively; and that the first particularcolor be a first chromatic color. It is also preferred that the secondparticular color be either black or a second chromatic color.

All of the foregoing operational principles and advantages of thepresent invention will be more fully appreciated upon consideration ofthe following detailed description, with reference to the appendeddrawings, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective or isometric view of a first printingenvironment in which preferred embodiments of the present invention havebeen found satisfactory: it is a representative inkjet printer which canincorporate the apparatus and method of the present invention;

FIG. 2 is a like view, but enlarged, of a carriage having removablemulticolor print cartridges, usable in the FIG. 1 printer;

FIG. 3 is a like view, but still further enlarged and generally from therear, of an exemplary lower-resolution color inkjet print cartridge usedin the FIG. 1 and 2 preferred embodiment;

FIG. 4 is a like view of the FIG. 2 carriage, but without the printcartridges shown in FIG. 2;

FIG. 5 is a top plan of the FIG. 2 and 4 carriage;

FIG. 6 is a fragmentary elevation of the flex-circuit interconnect onthe carriage of FIGS. 4 and 5, with the interior carriage wallspartially cut away;

FIG. 7 is a schematic block diagram of electronics for the system ofFIGS. 1 through 6;

FIG. 8 is a somewhat schematic bottom plan--i.e., as seen looking upfrom the printing medium--of the FIG. 7 nozzle arrays, showing theiralignment relationships;

FIG. 9 is a somewhat schematic partially exploded perspective orisometric view of a higher-resolution black inkjet pen cartridge of theFIG. 2 carriage system, together with a corresponding flex-circuittermination, and a mounting bracket that is a fragment of the FIG. 2carriage--particularly showing the use of a foam member for operativelyconnecting the flex-circuit to the cartridge;

FIG. 10 is a view like FIG. 1, but of a second printingenvironment--another representative inkjet printer in which the presentinvention can be incorporated--drawn with the cover and certain othercomponents broken away;

FIG. 11 is a view like FIG. 3, but of a so-called "trichamber" pen, adifferent type of pen or print cartridge that has within a single penbody reservoirs for ink of three different colors, and correspondingsets of ink-ejection nozzles for printing in those three colors, andparticularly for use in the FIG. 10 printer;

FIG. 12 is a like view but from the front and below, and particularlyshowing the three sets of nozzles, i.e. the nozzle array or arrays, ofthe FIG. 11 trichamber pen;

FIG. 13 is an enlarged elevation, analogous to a segment of FIG. 6, butrepresenting the FIG. 11 pen portion which carries a flex-circuitinterconnect;

FIG. 14 is a greatly enlarged bottom plan of the FIG. 12 nozzle arrays;

FIG. 15 is a like bottom plan, but somewhat less greatly enlarged, ofthe two pens in the FIG. 10 printer;

FIG. 16 is a top plan of portions of the FIG. 10 printer andparticularly showing in the phantom line how the footprints of the twoFIG. 15 pens are disposed together in relation to a sheet of printingmedium;

FIG. 17--analogous to FIG. 8--is a very greatly enlarged top plan of thenozzle plates in the FIG. 16 view, but drawn close together for easiercomparison;

FIG. 18 is a view analogous to FIGS. 1 and 10, but showing still a thirdprinting environment--a third representative inkjet printer in which thepresent invention is advantageously incorporated--and showing the coverclosed;

FIG. 19 is a view of the FIG. 18 printer but with the cover open;

FIG. 20 is a view, analogous to FIGS. 8 and 17, of nozzle arrays for twopens of the FIG. 18 printer--particularly including one trichamber penwith staggered nozzle arrays for three ink colors respectively, and oneblack-ink pen;

FIG. 21 is a highly schematic diagram of first exemplary relationsbetween swath heights and printing-medium advance according to theinvention, and particularly relating to the printer of FIGS. 1 through9;

FIG. 22 is a like diagram but particularly relating to the printer ofFIGS. 18 through 20;

FIG. 23 is a highly schematic diagram of an exemplary three-stageprintmask series, showing black and color inking under print-densitypractices according to the invention;

FIG. 24 is an exemplary firmware flow chart corresponding to the FIG. 23masking scheme--and also to those of FIGS. 25 through 27;

FIG. 25 is a diagram like FIG. 23 but comparing black and color inking,under resolution-controlled masking practices according to theinvention;

FIG. 26 is a diagram like FIGS. 23 and 25 but comparing inking fordifferent colors, under color-controlled masking practices consistentwith the invention; and

FIG. 27 is a like diagram for alternative color-controlled maskingconsistent with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. A First PreferredPrinting Environment

In one now-preferred embodiment of the invention disclosed herein, a23.6 dot/mm (600 dpi), 12.7-mm-swath (one-half-inch-swath) black pen iscombined with three 11.8 dot/mm (300 dpi) color pens each generating aswath approximately 8.5 mm (one-third inch) tall. The invention isuseful for applying text or graphics, or both, to media usingmonochrome, color, or mixed monochrome and color components.

The high-performance black-ink-dispensing pen is typically used forprinting text and other "black only" features, and thus the outputquality and throughput of these features is greater. It also improvesthe output quality of color graphics and color features by teaming withthe three lower-performance color-ink pens when printing color graphicsor color features.

The black component of the graphics, which is often a large portion ofcolor graphics content, is at a higher resolution and thus at a higheroutput-quality level. The taller swath can then be combined withprinting algorithms to improve the throughput of color graphics asdescribed in this document.

The printer 10 (FIG. 1) has an input tray 12 containing sheets ofprinting medium 14, which pass through a printing zone and along aprinting-medium advance direction 3 past an exit 18 into an output tray16. Electronic controls 9, 9' for commanding a microprocessor within theprinter to perform various functions, and mechanical controls 9" forrestraining and adjusting the printing-medium supply in the advancedirection, are included.

A movable carriage 20 (FIGS. 1 and 2) holds print cartridges 22, 24, 26,and 28 which respectively hold yellow (Y), magenta (M), cyan (C) andblack (K) inks--and dispense these inks upon command from amicroprocessor within the printer. The back of the carriage 20 hasmultiple bushings such as 34 which ride along a slide rod 36, enablingbidirectional movement of the carriage along the rod.

The carriage 20 thus moves along a carriage-scanning direction 2, abovea sheet of printing medium 14' (FIG. 1) upon which an image is beingformed by the print cartridges 22-28. The front of the carriage 20 has asupport bumper 30 which rides along a guide 32.

The position of the carriage, as it traverses the medium back and forth,is determined from an encoder strip 38 (FIG. 1). This very accuratepositioning enables selective firing of the various ink nozzles on eachprint cartridge at the appropriate times during each carriage scan.

A respective 11.8 dot/mm (300 dpi) color inkjet cartridge 40 (FIGS. 3through 6, also identifiable as any one of the three color-ink printcartridges 22, 24, 26) having a tab circuit with a four-column,thirty-two pad electrical interconnect 42 is removably installed in eachof three chutes 44, 46, 48 of a unitary carriage 20. A flex-circuitmember 52 having three matching sets of conductive pads 54, 56, 58 ismounted on flex-frame pins 60, to operatively engage the cartridge padsas each cartridge is inserted into its appropriate chute.

An enlarged set of conductive pads 62 covering a larger area, having adifferent layout, and constituting an array of six columns totalingfifty-two conductive pads on the flex-circuit member, is designed foroperative engagement with cartridge pads on a 23.6 dot/mm (600 dpi)black inkjet cartridge 64 (FIG. 9, also identifiable as print cartridge28 of FIGS. 1 and 2).

Preferred structure and techniques for preventing mistaken installationof a 23.6 dot/mm black-ink printhead in a color-ink printhead chute--oralternatively the mistaken installation of an 11.8 dot/mm color-inkprinthead in a black-ink printhead chute--are described in the copendingapplications identified above and incorporated by reference into thisdocument.

Because of the differently configured electrical interconnect on the23.6 dot/mm (600 dpi) cartridge 28, and in order to avoid substantiallychanging the existing X/Y/Z datum configuration of the carriage, aunique interconnect scheme is employed. Details appear in the Harris etal. document and related copending applications identified above andincorporated by reference herein.

As suggested in FIG. 9, for satisfactory performance particularattention and care must be given to proper alignment of the black-inkcartridge 28 relative to the other cartridges and relative to theprinting medium 14', and full contact of the largerelectrical-interconnect surface area 52--with its greater number ofindividual contacts. It has been found that this objective is metparticularly well through incorporation of a unique spring assembly forthe 600 dpi cartridge interconnect.

This assembly includes a unitary resilient foam biasing member 84 whichhas been found to importantly provide more-uniform interface connectionpressure over the full interconnect area. The foam biasing member 84fits in a seat 86 (FIG. 9), which is formed as part of the carriagecradle.

A mounting peg 88, protruding from the seat 86, fits into a matchinghole 90 in the foam member 84. This interfit, along with bottom andlower ledges 91, 93 and upper side and top ledges 92, 94 of the seat 86,holds the foam member 84 in proper position to assure operativeengagement across the electrical interconnect 52.

For the shallower-swath, lower-resolution color-ink pens, with theirsmaller number of interface connections, a more-conventional metallicspring (not shown) has been found entirely adequate to ensure reliablecontact at all the connection points. Once again, the documentsmentioned above present fuller details, particularly for the structureand function of the flex-circuit frame, which has been modified for thetaller print cartridge 28.

Preferred mounting relationships between an 11.8 dot/mm (300 dpi) nozzlearray 96 (FIGS. 7 and 8) of the color-ink-dispensing printheads and a23.6 dot/mm (600 dpi) nozzle array 98 of the taller,black-ink-dispensing printhead 28 are also important to satisfactoryperformance. Control circuitry 99 on the substrate includes multiplexingto enable the three hundred firing resistors 97 of the black-inkprinthead to be controlled through fifty-two electrical interconnectpads 62.

Analogous features enable all one hundred four firing resistors 97' ofeach color-ink printhead to be controlled through thirty-two electricalinterconnect pads. The multiplexing circuit scheme for such controlcircuitry is described more fully in the other documents identifiedabove and incorporated herein by reference.

FIG. 8 shows very schematically side by side the nozzle plate 98 of theblack-ink pen 28 and a representative nozzle plate 96 of one (22, 24 or26) of the three color-ink pens. FIG. 8 is not to scale, particularlywith respect to nozzle spacing 5, 7 or its reciprocal, nozzle pitch, foreither the color- or black-ink plate 96, 98.

Actually along the medium-advance axis 3 the nozzle spacing 7 in theblack pen is half the nozzle spacing 5 in the color pen. Thereforewithin the 8.5 mm (1/3 inch) swath height 6 (of the color pen) indicatedin the drawing are for example one hundred nozzles #1 through #100 ofthe color-ink nozzle plate 96--and two hundred nozzles, for example #51through #250, of the black-ink plate 98.

Accordingly the remaining 300-200=100 nozzles of the black-ink plate areoutside the 8.5 mm color swath height 6. In the printer configurationillustrated here by way of example, these two pens are mutually centeredabout a common centerline 9, so that--of the hundred black-ink nozzles93 outside the 8.5 mm color swath height 6--fifty nozzles #1 through #50are above and fifty nozzles #251 through #300 below.

Nozzles conventionally are numbered for addressing purposes. Black-pennozzles #1 through #50 are above (with respect to the "up" directionalong the printing medium) color-pen nozzle #1; and black-pen nozzles#251 through 300 are below color-pen nozzle #100.

As noted above the foregoing discussion refers to a very schematicrepresentation (FIG. 8). A preferred color-ink printhead has one hundredfour resistors and nozzles rather than one hundred. Some relationsdescribed above are simplified for explanatory purposes, and merelyrepresentative of actual preferred numbers and alignments.

In one particularly preferable form, one hundred four color-ink nozzlesare present. As their swath encompasses two hundred eight of the threehundred black-ink nozzles, only ninety-two black-ink nozzles are outsidethe color swath. Forty-four are above, and forty-eight below.

All the color- and black-ink nozzles within the color-ink swath areusable together, for highest throughput; however, for certain aspects ofthe present invention (particularly low pixel density) it is better toactually use only ninety-six of the one hundred four colornozzles--encompassing one hundred ninety-two black-ink nozzles.

This preference comes from practical concerns such as number of nozzlesper swath. Based on disclosures herein, such concerns will be clear tothe skilled artisan.

2. A Second Preferred Printing Environment

Many or most of the features described in the immediately precedingsubsection are equally applicable here. Accordingly this discussionfocuses on some of the-more-salient differences.

Features of this second printing environment, shown in FIGS. 10 through16, which correspond to features of the first printing environment inFIGS. 1 through 9, have been given like reference numerals but with adistinguishing prefix "1". (Hence for example the black-ink pens 98 and198.)

Whereas four separate, discrete pens 22, 24, 26, 28 (FIG. 1) are in thefirst printer, only two pen bodies 122, 128 (FIG. 10) are in this secondprinter. One pen body 122, however, is wider than the other pen body 128and--compared with the separate color pens 22, 24, 26 discussedearlier--has a relatively large number of individual connections in itsinterface pad 142 (FIGS. 11 through 13).

This pen body also has not one but three arrays 143a, 143b, 143c ofink-ejecting nozzles (FIGS. 12, 14 and 15). Each array is a doublecolumn having thirty-two nozzles in each column, for a total ofsixty-four in each array and one hundred ninety-two overall.

The color-ink nozzles are defined in an orifice plate or nozzle plate196, oriented parallel to and facing the printing medium 114'. FIG. 17shows this nozzle plate 196 and the plate 198 of the other pen 122,together--not as they would normally appear from below, but rather inmirror image, or as seen through the tops of the cartridges 122, 128looking toward the print medium 114'.

For orientation purposes a like point of view is assumed in FIG. 16,which is greatly reduced relative to FIG. 17. FIG. 16 also shows inkswaths 106, 108 produced on the medium 114' by the two pens 128, 198respectively.

In addition FIGS. 10 and 16 show portions 101 of the drive mechanismthat advances the printing medium 114, 114' along an advance direction103. These elements 101 are representative as well of medium-advancedrive components (not shown) in the other two printing environmentsdiscussed in this document. The print-medium advance drive mechanism iscontrolled by the same programmed microprocessor (not shown, but see theanalogous "main processor board" 49, FIG. 7, for the above-discussedfirst printing environment) which controls the pens.

Within the pen body 126, each of the three double-column arrays 143a-cis connected--through individual nozzle firing devices (not illustrated,but the electronic portions of which are analogous to resistors 97' inFIG. 7)--to a respective reservoir (not shown) of differently coloredink, so that in operation the nozzles of each array when actuated emit adifferent color of ink (typically yellow, magenta and cyanrespectively). The arrays are mutually parallel, and parallel to thedirection of print-medium advance--i.e., the swath-height direction.

In each color array, adjacent nozzles are alternately offset to form azigzag pattern, providing the benefit of close nozzle spacing along theswath-height direction without physical interference between adjacentnozzles. Thus the odd-numbered nozzles of each array are in one columnwhile the even-numbered nozzles of the same array are in a second,parallel column.

In a preferred embodiment, the spacing between these columns oflike-color nozzles is 2/3 mm (0.027 inch), and the color arrays arespaced apart from each other by 21/2 mm (0.1 inch). Control softwaretimes the output of the nozzles to compensate for their differentlateral positions.

The nozzle pitch of each color array--i.e., the reciprocal of the nozzlespacing 105 (FIG. 17)--is 11.8 nozzles/mm (300 nozzles/inch). Dividinginto sixty-four, the number of nozzles in each array, the total lengthof each array is thus 5.4 mm (0.21 inch).

The black-ink printhead 128 is substantially identical to that 98described above for the printer of FIGS. 1 through 9. The black-inknozzle array 193 is also arranged in two columns of alternating nozzles,in this case spaced about 4.1 mm (0.16 inch) apart. Here the nozzlepitch--the reciprocal of the nozzle spacing 107, FIG. 17--is 23.6nozzles/mm (600 nozzles/inch).

In all plural-pen systems, accurate mutual positioning of the two ormore nozzle plates is important. The mechanical registration points orso called "datum" surfaces are carefully arranged to ensure precisealignment. The even-numbered nozzle centers of the black-ink pen arecentered along a line positioned to the right of the even-numberednozzle centers of the rightmost color set by about 7.4 mm (0.29 inch).

In this system, unlike the first printing environment discussed earlier,the black-ink pen is disposed asymmetrically with respect to thecolor-ink pen (FIG. 17). Of the three hundred black-ink nozzles 193, onehundred twenty-nine nozzles 108" (nozzles #1 through #129) extend beyondthe color-ink nozzles in the direction 103 of print-medium advance; butonly forty-five nozzles 108' (#256 through #300) in the oppositedirection.

Such asymmetry can be used for various purposes, as for example to helpcontrol adverse interactions between different inks, or between inks andthe printing medium--as set forth in the contemporaneously filedapplications, previously enumerated, of Mark Stephen Hickman.

Neglecting nozzle diameter, the color swaths 106 extend from upper colorlimit 106' to lower color limit 106" employing color nozzles #1a-cthrough #64a-c; and this color swath encompasses one hundred twenty-sixblack-ink nozzles #130 through #255.

3. A Third Preferred Printing Environment

As in the two cases already discussed, like reference numerals are usedfor corresponding features in this third case--but with a prefix "2".Thus for example the printer generally is designated 210, FIGS. 18 and19, in correspondence with the printers 10, 110 of the earlierdiscussions.

This printer too has adjoining cradles in a carriage 220 (FIG. 19) for ablack-ink pen represented by nozzle plate 298 (FIG. 20) and a trichambercolor-ink pen represented by nozzle plate 296. Here the emphasis is oneconomy, as the black-ink pen 226 has only forty-eight nozzles and thecolor-ink pen 228 just sixteen for each color. Resulting print speed islower.

The color-ink nozzle arrays 243a, 243b, 243c of this trichamber pen arestaggered, with one array 243b (typically for magenta) offset to theright of the other two (typically for yellow and cyan). Nozzle spacing205 in the black-ink and color-ink arrays is the same, 0.085 mm (1/300inch), yielding coarser resolution for black than in the previouslydiscussed systems.

The height 206 of each color-ink array 243a-c is thus about one-thirdthe height 208 of the black-ink array. About 0.4 mm (1/60 inch) addedclearance 74, 75 is provided between the color-ink arrays, in theswath-height direction, making the overall combined height of the threestaggered color-ink nozzle arrays just slightly (0.8 mm or 1/30 inch)greater than the height of the black-ink array.

In this system the black-ink pen is disposed almost symmetrically withrespect to the color-ink pen (FIG. 20). Three color-ink nozzles (#1through #3 of the topmost array 243a) extend above the black-ink nozzlearray 293, and five nozzles (#12 through #16 of the bottom-most array243c) below--for an asymmetry amounting to two nozzle-spacing distancesor just 1/6 mm (1/150 inch).

Due to the staggered-array configuration, this trichamber nozzle plate296 and its corresponding pen can be narrower than the trichamber pen196 of FIGS. 11 through 17. Each of the three double-column arrays243a-c (FIG. 20) has a zigzag pattern and is connected within the penbody 226, through individual nozzle firing systems (not shown but see97' in FIG. 7), to a respective reservoir (not shown) of differentlycolored ink, as in the system discussed previously.

In a preferred embodiment, the spacing 72 between individual nozzlecolumns is 2/3 mm (0.027 inch), as in the previous case. Here the colorarrays are mutually offset by a distance 73 of about 11/2 mm (0.06inch);

With forty-eight nozzles at 11.8 nozzles/mm (300 nozzles/inch), theblack-ink array is nominally 4.1 mm (0.16 inch) tall. The overallcolor-ink array is eight nozzle spacings taller, totaling about 4.8 mm(0.19 inch).

The black-ink nozzle array 293 too is arranged in two columns ofalternating nozzles, in this case spaced apart by a distance 71 of about0.85 mm (1/30 inch). In both pens the separation 211 between adjacentnozzles within each nozzle column is twice the effective nozzle spacing,or pixel spacing, 205.

4. Different Pixel Densities in an Extended Printzone

By "extended printzone" is meant a swath height 8, 108, 208 that isavailable using a first array 93, 193, 293 of colorant-dispensingdevices--and that is greater than a swath height 6, 106, 206 availablein concurrent printing using a second array 43a-c, 143a-c, 243a-c ofcolorant-dispensing devices.

In all three printing environments described above, the "first array" isthe array of black-ink-dispensing inkjet nozzles 93, 193, 293. The"second array" is any one of the three arrays of single-color-dispensinginkjet nozzles 43a-c, 143a-c, 243a-c.

The taller swath height 8, 108, 208 allows any given amount of ink to bespread over a taller area. For example, if for some particular image itis desired to print equal amounts of yellow and black ink, the tallerblack-ink pen can distribute black ink over a taller area than theyellow-ink pen--while the printing-medium 14', 114', advances in keepingwith the print rate of the shallower, yellow-ink pen.

More generally in printing with all colors the taller head distributesink over a taller area while the print-medium advance keeps pace withthe shallower head. This allows the taller printhead to put down ink ata slower rate, during a greater number of passes--while still producingthe same amount of total coverage, after all the passes are complete, asthe shallower heads.

For instance suppose that the taller nozzle array A (FIG. 21), whichdischarges a corresponding color A, has a height 8 of three units; andthe shallower array B, discharging color B, has a height 6 of two units.The taller array A can use three passes to coat color A over an areawith a swath advance of one unit, while the shallower array B puts downthe same amount of ink of color B in two passes.

FIG. 23 shows this process for color A from a three-unit-tall array A,and for color B from a two-unit-tall array B--the relationship of FIG.21. Inking of color A in the first pass P1 is symbolized as A1, in thesecond pass P2 as A2 and in the third pass P3 as A3. Inking of color Bin each of two passes P1, P2 is symbolized as B1 and B2 respectively.

In one of the three passes used for color A, no inking of color Boccurs: here it is the third pass P3, but it could be the first P1instead, depending e.g. on the direction in which the extended printzoneis made to extend. FIG. 23 also shows that the taller pen A can ink at alower density DA of 33% (i.e., one-third) per pass, in each of its threepasses, compared with 50% density D_(B) per pass for two passes.

This invention thus allows ink to be spread over a wider area whenprinting with two or more colors, to improve various print attributessuch as drying time, bleed, cockle and curl of the printing medium, etc.For printing with only the extended-printzone color, however, theinvention enables increases in throughput without increasing the size ofall the inking arrays.

FIG. 24 shows how the FIG. 23 masking can be implemented in firmware.Those skilled in the art will find the flow chart of FIG. 24 selfexplanatory.

FIG. 24 is only exemplary; those skilled in the art of programming forpixel-based printing machines will understand that many other schemescan be employed equivalently for causing a programmed microprocessor toproduce the masking of FIG. 23.

5. Resolution- or Color-Dependent Masking

FIG. 25 shows inking by a higher-resolution pen A at left, and alower-resolution pen B at right. The smaller numerals at left representfiner pixels 77_(A), and in both parts of the drawing "1" and "2"represent a first and second pass respectively.

The areas covered by the two pens A, B are understood to be mutuallyaligned at the upper left-hand corners of the two patterns. Thehigher-resolution pen A, at-left, has pixel cells 77_(A) half the sizeof the pixel cells 77_(B) for the lower-resolution pen at right.

Since the dimensions of the finer and coarser pixels 77_(A), 77_(B)stand in a ratio of two small integers (namely 1:2), they could easilybe printmasked in common. That is to say, the finer pixels 77_(A) of thedevice A at left could be masked in two-pixel-square groups e.g. 77_(A)', and these two-pixel-square groupings 77_(A) ' given exactly the samemasking treatment as individual pixels 77_(B) of the coarser-pixeldevice B at right.

In short, wherever a pixel 77_(B) ' was printed by the coarse-resolutiondevice B at right, four pixels 77_(A) ' would be printed by thefine-resolution device A at left.

The invention, however, provides different masking for the two cases.The finer-pixel device A at left is masked in clusters 78_(A) that aremismatched relative to the coarser pixels at right, instead of either aneven distribution (i.e., every other pixel printed in each pass) or adistribution 77_(A) ' that matches the masking 77_(B) ' for thecoarser-pixel device B at right.

FIG. 25 illustrates a choice of three-pixel squares 78_(A) for maskingof the left-hand finer-pixel device A, and also shows two-by-one-pixeltall rectangles 78_(B) for the right-hand coarser-pixel device B. As canbe seen by inspecting the diagram, the result of this combined maskingpattern is relatively irregular conjunction of the two patterns 78_(A),78_(B).

The cross-combination pattern units tend to be odd-shaped, and to repeatat different intervals in orthogonal directions. These resultingirregular patterns interact differently with dot placement errors toprovide an even more erratic, or less evidently patterned, field ofoutput colors.

FIG. 24, already mentioned above, is also applicable to show how theFIG. 25 masking may be implemented through firmware. FIG. 24 is in factgeneral to these systems and applies as well to the remainingembodiments of FIGS. 26 and 27 discussed below.

FIG. 25 illustrates different masking for different resolutions. FIGS.26 and 27 illustrate a related innovation: different masking fordifferent colors.

Although the two embodiments can be combined, FIGS. 26 and 27 showcommon resolution for the two colors illustrated--which will be the caseas between chromatic colors, in all three of the printing environmentsintroduced above. In FIGS. 26 and 27 the prefixes "M" and "C" representdifferent chromatic colors (typically magenta and cyan respectively),and as before the suffixes "1" and "2" represent first and second passesrespectively.

FIG. 26 shows one masking pattern (rectangles 79_(A), each two pixelstall by one pixel wide) for magenta, and another pattern(two-by-two-pixel squares 79_(B)) for cyan. FIG. 27 shows what may beregarded as a converse arrangement (rectangles 79_(A) ' one pixel tallby two pixels wide for magenta). A simpler example (not illustrated) isuse of the bottom half of one pen and the top half of another.

While the two pens scan across the printing medium, both colors are putdown at the same time but on different areas of the medium. If thecolors are mixed on any given pixel, to create a secondary color, theprocedure described tends to allow the first ink to dry and penetrateinto the paper--before the second color is placed on top of or next tothe first.

This technique enhances print quality by minimizing interactions betweeninks, as well as cockle, curl, bleed etc., while minimizing the need toset aside time for drying. The overall result is greater independence ofprinting media, and better print quality.

The exemplary patterns of FIGS. 25 through 27 have been foundparticularly beneficial for the first printing environment introducedabove. Within this regimen as provided by the invention, some trial anderror may be helpful in selecting ideal masking for a particularcombination of resolutions and other pen parameters.

The above disclosure is intended as merely exemplary, and not to limitthe scope of the invention--which is to be determined by reference tothe appended claims.

What is claimed is:
 1. A single pass mixed-masking printer for formingimages as an assemblage of ink dots at pixel locations on a printmedium; said printer comprising:a high resolution print head mounted forprinting an image swath on a particular region of the print medium; alow resolution print head mounted for concurrently printing anotherimage swath on the same particular region of the print medium; aprocessor coupled to said high resolution print head for imposing onsaid image swath a printmask; and said processor further coupled to saidlow resolution print head for concurrently imposing on the another imageswath another printmask that is different from said printmask.
 2. Theprinter of claim 1, wherein:said high resolution print head has a firstnozzle pitch; and wherein said low resolution print head has a secondnozzle pitch that is different from the first pitch.
 3. The printer ofclaim 2, wherein:the first nozzle pitch is twice as fine as the secondnozzle pitch.
 4. The printer of claim 1, wherein:a printmask cell forsaid low resolution printhead is related to a printmask cell for saidhigh resolution printhead by a relationship which is selected from thefollowing group of relationships: the printmask cell for said lowresolution printhead is wider than the printmask cell for said highresolution printhead; and the printmask cell for said low resolutionprinthead is taller than the printmask cell for said high resolutionprinthead.
 5. A method of forming images as an assemblage of ink dots atpixel locations on a printing medium; said method comprising the stepsof:printing on a particular region of the medium, using a firstprintmask, an image swath of a first particular character; concurrentlyprinting on the said particular region of the medium, using a secondprintmask that is different from the first printmask, an image swath ofa second particular character which is different from the firstparticular character; wherein said first and second particularimage-swath character differ in regard to a parameter selected from thegroup consisting of:resolution; and color.
 6. The method of claim 5,wherein:said image swath of a first particular character and said imageswath of a second particular character comprise a first and secondparticular color respectively; the first particular color is a firstchromatic color; and the second particular color is selected from thegroup consisting of:black, and a second chromatic color.
 7. A singlepass mixed-masking printer, comprising:a plurality of low resolutionprintheads aligned about a common horizontal carriage axis for ejectingdifferent color droplets of ink onto a print medium surface in a commoncolor ink swath having a given height; a single high resolutionprinthead aligned with said plurality of low resolution printheads aboutsaid common axis for ejecting black ink droplets onto the print mediumsurface in a black ink swath having another given height; and saidanother given height of said black ink swath being substantially greaterin height than said given height of the color ink swath so that in ahigh performance mode of operation, the black ink droplets ejected inthe common color ink swath can be mixed with the color ink dropletsejected in the common color ink swath to improve color graphicsthroughput.
 8. A single pass mixed-masking printer, comprising:a singlelow resolution printhead having a plurality of groups of ink ejectingnozzles aligned about a common horizontal carriage axis for ejectingdifferent color droplets of ink onto a print medium surface in a commoncolor ink swath having a given height; a single high resolutionprinthead having a single group of ink ejecting nozzles aligned aboutsaid common horizontal carriage axis for ejecting black ink dropletsonto the print medium surface in a black ink swath having another givenheight; and said another given height of said black ink swath beingsubstantially greater in height than said given height of the color inkswath so that in a high performance mode of operation, the black inkdroplets ejected in the common color ink swath can be mixed with thecolor ink droplets ejected in the common color ink swath to improvecolor graphics throughput.